A typical optical receiver (Rx) includes at least one photodiode that detects an optical signal and converts it into an electrical current signal and at least one transimpedance amplifier (TIA) that converts the electrical current signal into an electrical voltage signal. The photodetector, which is typically a P-intrinsic-N (PIN) photodiode, produces an electrical current signal in response to light detected by the photodetector. The TIA converts this electrical current signal into an output voltage signal having some gain, commonly referred to as transimpedance gain. This output voltage signal is further processed by other circuitry of the optical Rx (e.g., a limiting amplifier (LA), clock and data recover (CDR), etc.).
The output terminal of the photodiode is connected to the inverting input terminal of the TIA. Due to mismatch in the input stage and/or a systematic input bias voltage of the input stage, an input offset voltage is generated. The offset in the input voltage of the TIA results in an offset in the output voltage of the TIA. The common approach that is used to reduce or eliminate the offset is to use larger transistors in the input stage of the TIA. This approach, however, increases noise and therefore reduces the sensitivity of the optical Rx. Often, a tradeoff has to be made between the amount of noise that will be tolerated and the magnitude of the offset that will be tolerated. In addition, because the offset has a random nature, the offset output voltage together with noise can falsely trigger the decision circuitry of the Rx. For this reason, the decision threshold of the Rx is usually provided with some headroom to prevent noise from falsely triggering the decision circuitry of the Rx under worst case offset conditions.
In addition, applications with multi-mode or single-mode fibers may demand decreasing photodiode diameters to lower photodiode capacitances and therefore to improve sensitivity. As a result, the optimum noise figure achieved through noise matching is only possible if smaller transistors are used in the TIA. Furthermore, advancements in process technologies have also resulted in transistors having smaller gate areas, which in turn will result in even larger offsets.
Accordingly, a need exists for a way to eliminate or reduce the offset without increasing noise and/or without reducing the sensitivity of the Rx in which the TIA is employed.